os/exec: return clear error for missing cmd.Path

os/exec: return clear error for missing cmd.Path

Following up on CL 403694, there is a bit of confusion about
when Path is and isn't set, along with now the exported Err field.
Catch the case where Path and Err (and lookPathErr) are all unset
and give a helpful error.

Fixes #52574
Followup after #43724.

Change-Id: I03205172aef3801c3194f5098bdb93290c02b1b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/403759
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>